This invention relates generally to computed tomography (CT) imaging and more particularly to methods and apparatus for generating CT imaging data using a multi-slice imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reducing scan time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
In known CT systems the x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray-absorbing material with an aperture therein for restricting the x-ray beam.
By imposing limitations on such factors as collimator aperture size and slice thickness, CT imaging systems provide image resolution. A collimator defines the dimensions of the x-ray beam(s), particularly its width. A collimator""s aperture width typically controls the slice thickness as measured along the z-axis. Known apertures are typically linear or rectangular. By defining x-ray beam width, a collimator determines the thickness of an individual slice or group of slices. By reducing the slice thickness, the image resolution is improved. For example, by passing an x-ray beam through a collimator with a 1 millimeter aperture, the beam output from the collimator will have a 1 millimeter thickness.
Known CT systems typically utilize collimators having at least one 1 mm aperture. While 1 millimeter or higher slices are effective for many CT system applications, in some CT system applications, a thinner slice thickness is desired. Particularly, in some applications, it is desirable to generate an image with, for example, submillimeter slice images. Such smaller slice images are specifically desirable when patient anatomy differs in areas less than 1 millimeter apart.
One way to obtain thinner slices is to make the detector cells thinner. However, this requires a great amount of hardware redesign and also requires sacrificing scanner coverage and speed in most applications. A minimum slice thickness for at least one CT system is 1.25 millimeters, as determined primarily by detector element pitch size. In order to improve image resolution, it is desirable to reduce slice thickness to less than 1 millimeter. In some applications, a slice thickness as thin as 0.5 millimeter is desired.
It is known to reduce slice thickness of a single-slice imaging system by irradiating a portion of a detector element and deconvolving imaging data to reduce the full-width-at-half-maximum (FWHM) interval of a reconstructed slice profile. It is desirable to achieve similar slice-width reductions on multi-slice systems without reducing coverage. However, difficulties arise in implementing this approach for a multi-slice imaging system because multi-slice sampling is limited by joints between adjacent detector rows.
It would be desirable to improve image resolution in a multi-slice CT system by providing a slice thickness less than 0.5 mm, or submillimeter slices, by using single-slice imaging data collection with multiple x-ray source collimators, different sampling schemes and deconvolution techniques.
There is therefore provided, in one embodiment of the present invention, a method for imaging an object using a multi-slice computed tomography (CT) imaging system having a radiation source and a detector, the detector having a plurality of detector rows configured to acquire projection data from a scanned object between the radiation source and the detector, wherein each of the detector rows is perpendicular to and has a thickness in a z-direction. The method includes steps of collimating a radiation beam from the radiation source into a plurality of separate beam portions transverse to the z-direction so that the separate beam portions pass through the object and impinge on the detector rows; scanning the object using the plurality of separate beam portions to acquire projection data; and reconstructing an image of the object utilizing the acquired projection data.
The above described embodiment provides clinically useful submillimeter scan modes with greatly increased coverage, compared to known methods.